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Difficulty: HardIron: Blast Furnace Extraction, Rusting Prevention, and Chemical Compounds

Arrange the following chemical and electrochemical stages of the rusting of iron in chronological sequence, from initial anodic oxidation to the final formation of rust.

  1. 1Oxidation of metallic iron at anodic regions to yield iron(II) ions (Fe2+\text{Fe}^{2+}) and free electrons.
  2. 2Reduction of dissolved oxygen and water at cathodic regions by released electrons to form hydroxide ions (OH−\text{OH}^-).
  3. 3Combination of Fe2+\text{Fe}^{2+} and OH−\text{OH}^- ions in the moisture layer to precipitate green iron(II) hydroxide (Fe(OH)2\text{Fe(OH)}_2).
  4. 4Further oxidation of iron(II) hydroxide by dissolved oxygen to produce reddish-brown hydrated iron(III) oxide (Fe2O3⋅xH2O\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O}).

Answer

The correct chronological sequence is: Oxidation of iron metal to Fe2+\text{Fe}^{2+} ions →\rightarrow Reduction of dissolved oxygen to OH−\text{OH}^- ions →\rightarrow Precipitation of Fe(OH)2\text{Fe(OH)}_2 →\rightarrow Oxidation of Fe(OH)2\text{Fe(OH)}_2 to hydrated Fe2O3⋅xH2O\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O}.
Rusting is an electrochemical process. Iron metal initially oxidizes at anodic sites to produce Fe2+\text{Fe}^{2+} ions and electrons. The released electrons migrate through the iron to cathodic regions, where dissolved atmospheric oxygen is reduced to OH−\text{OH}^- ions. These ions react in the electrolyte solution to form insoluble green Fe(OH)2\text{Fe(OH)}_2, which is subsequently oxidized by dissolved oxygen to form reddish-brown hydrated iron(III) oxide (rust, Fe2O3⋅xH2O\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O}).

Step-by-Step Solution

1
Identify the initial anodic oxidation process
Fe(s)→Fe2+(aq)+2e−\text{Fe(s)} \rightarrow \text{Fe}^{2+}\text{(aq)} + 2\text{e}^-
Rusting begins as an electrochemical reaction where iron metal acts as the anode and undergoes oxidation.
2
Identify the cathodic reduction process
O2(g)+2H2O(l)+4e−→4OH−(aq)\text{O}_2\text{(g)} + 2\text{H}_2\text{O(l)} + 4\text{e}^- \rightarrow 4\text{OH}^-\text{(aq)}
Electrons released during anodic oxidation flow to cathodic sites where atmospheric oxygen dissolved in water is reduced.
3
Determine the ionic precipitation reaction
Fe2+(aq)+2OH−(aq)→Fe(OH)2(s)\text{Fe}^{2+}\text{(aq)} + 2\text{OH}^-\text{(aq)} \rightarrow \text{Fe(OH)}_2\text{(s)}
The formed cations and anions diffuse towards each other in the aqueous electrolyte layer, forming an insoluble precipitate.
4
Determine the final oxidation step to rust
4Fe(OH)2(s)+O2(g)+2xH2O(l)→2(Fe2O3⋅xH2O)(s)4\text{Fe(OH)}_2\text{(s)} + \text{O}_2\text{(g)} + 2x\text{H}_2\text{O(l)} \rightarrow 2(\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O})\text{(s)}
Dissolved oxygen further oxidizes iron(II) hydroxide to hydrated iron(III) oxide, commonly known as rust.

Key Concept

Electrochemical mechanism of iron rusting
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Topics

Iron: Blast Furnace Extraction, Rusting Prevention, and Chemical Compounds

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